Infrared detector, imaging device, and imaging system

ABSTRACT

An infrared detector includes a quantum dot structure, and an electrode that is coupled to the quantum dot structure, wherein the quantum dot structure is obtained by stacking a plurality of structures each including a quantum dot, a first barrier layer under the quantum dot and a second barrier layer over the quantum dot to cover the quantum dots, and an intermediate layer under the first barrier layer, and wherein the first barrier layer includes a first region and a second region having a lower Al concentration than that of the intermediate layer between the first region and the intermediate layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2017-137313, filed on Jul. 13,2017, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an infrared detector, animaging device, and an imaging system.

BACKGROUND

Currently, a quantum dot type semiconductor device using quantum dots isactively researched. As one of the quantum dot type semiconductordevices, there is a quantum dot infrared photodetector (QDIP) which isoperated by exciting carriers confined in quantum dots when beingirradiated with infrared rays so as to be detected as a photoelectriccurrent. Regarding a structure of the QDIP, various researches areconducted.

As an infrared detector which realizes desired long wavelengthcharacteristics and has a low dark current and a high sensitivity, astructure in which an upper part and a lower part of quantum dots arecovered with a barrier layer such as AlAs is proposed. However, in thiscase, impurities are incorporated into the quantum dots due to Al of thebarrier layer at the time of forming the quantum dots, and an impuritylevel is formed in the quantum dots. Therefore, there is a problem inthat noise increases and an S/N ratio decreases when detecting infraredrays.

The followings are reference documents.

-   [Document 1] Japanese Laid-open Patent Publication Nos. 2009-65141,-   [Document 2] Japanese Laid-open Patent Publication No 2012-195333,    and-   [Document 3] Japanese Laid-open Patent Publication No 2016-136585.

SUMMARY

According to an aspect of the invention, an infrared detector includes aquantum dot structure, and an electrode that is coupled to the quantumdot structure, wherein the quantum dot structure is obtained by stackinga plurality of structures each including a quantum dot, a first barrierlayer under the quantum dot and a second barrier layer over the quantumdot to cover the quantum dots, and an intermediate layer under the firstbarrier layer, and wherein the first barrier layer includes a firstregion and a second region having a lower Al concentration than that ofthe intermediate layer between the first region and the intermediatelayer.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are schematic sectional views illustrating amanufacturing method of a QDIP in order of processes according to afirst embodiment;

FIGS. 2A to 2C are schematic sectional views illustrating themanufacturing method of the QDIP in order of processes according to thefirst embodiment following FIGS. 1A to 1C;

FIGS. 3A and 3B are schematic sectional views illustrating themanufacturing method of the QDIP in order of processes according to thefirst embodiment following FIGS. 2A to 2C;

FIGS. 4A and 4B are schematic sectional views for describingincorporation of Al in a forming process of quantum dots;

FIG. 5 is a characteristic diagram illustrating S/N ratios of acomparative example and the first embodiment;

FIGS. 6A to 6C are schematic sectional views illustrating a main processin a manufacturing method of a QDIP in order of processes according to asecond embodiment;

FIGS. 7A and 7B are schematic sectional views illustrating the mainprocess in the manufacturing method of the QDIP in order of processesaccording to the second embodiment following FIGS. 6A to 6C;

FIGS. 8A and 8B are schematic sectional views illustrating a mainprocess in a manufacturing method of a QDIP in order of processesaccording to a third embodiment;

FIG. 9 is a schematic sectional view illustrating the main process inthe manufacturing method of the QDIP in order of processes according tothe third embodiment following FIGS. 8A and 8B;

FIG. 10 is a perspective view illustrating a schematic configuration ofan infrared imaging device according to a fourth embodiment;

FIG. 11 is a schematic sectional view illustrating an enlarged part ofthe infrared imaging device according to the fourth embodiment; and

FIG. 12 is a schematic diagram illustrating a schematic configuration ofan infrared imaging system according to a fifth embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

Hereinafter, a first embodiment will be described. In the embodiment, aquantum dot infrared photodetector (QDIP) is disclosed as an infrareddetector, and a configuration thereof will be described with amanufacturing method.

FIGS. 1A to 3B are schematic sectional views illustrating amanufacturing method of the QDIP in order of processes according to thefirst embodiment.

Each layer of the QDIP is formed by, for example, an epitaxial growthmethod using a molecular beam epitaxy (MBE). Also, a metal organicchemical vapor deposition (MOCVD) or the like may be also used. Quantumdots of the QDIP are formed by, for example, a self-assembly due to alattice mismatching.

First, as illustrated in FIG. 1A, a lower contact layer 2 is formed on aGaAs substrate 1.

In detail, for example, the GaAs substrate 1 is prepared as a substrateand is introduced into a loadlock chamber of an MBE. The GaAs substrate1 is outgassed in a preparation chamber. After that, the GaAs substrateis transfered to a growth chamber which is maintained as an ultrahighvacuum chamber. The GaAs substrate 1 transfered to the growth chamber isheated under As overpressures in order to remove an oxide film on asurface of the substrate. After the oxide film is removed, in order toimprove flatness of the surface of the substrate, for example, a GaAsbuffer layer (not illustrated) is grown to, for example, approximately100 nm at approximately 600° C. of a substrate temperature.

Next, as, for example, 600° C. of the substrate temperature, forexample, Si doped n-type GaAs having an electron concentration1×10¹⁸/cm³ is grown to a thickness of approximately 250 nm. Accordingly,the lower contact layer 2 is formed on the GaAs substrate 1. Here,n-type impurities other than Si may be also doped. In addition, acarrier is an electron, but a hole can be also used.

Subsequently, as illustrated in FIG. 1B, an intermediate layer 11 isformed.

In detail, a compound semiconductor material containing Al, for example,Al_(b)Ga_(1-b)As (0≤b<1), specifically, Al_(0.2)Ga_(0.8)As is grown to athickness of approximately 25 nm. Accordingly, the intermediate layer 11is formed on the lower contact layer 2.

Subsequently, as illustrated in FIG. 1C, a first barrier layer 13 isformed.

In detail, first, a compound semiconductor material having a compositionwith a lower Al concentration than that of the intermediate layer 11,for example, Al_(a)Ga_(1-a)As in which a<b (0≤a<0.2), here, GaAs notcontaining Al (a=0) is grown to a thickness of, for example,approximately 0.3 nm, on the intermediate layer 11. Therefore, a secondregion 12 b is formed.

Next, a compound semiconductor material having a composition with ahigher Al concentration than that of the intermediate layer 11, forexample, Al_(c)Ga_(1-c)As in which b<c (0.2<c≤1), here, AlAs notcontaining Ga (c=1) is grown to a thickness of, for example, 2 nm orless, here, approximately 0.3 nm, on the second region 12 b.Accordingly, a first region 12 a is formed. When the first region 12 ais formed to a thickness of 2 nm or less, the influence of theincorporation of Al on quantum dots 14 to be described later islessened.

Accordingly, the first barrier layer 13 including the first region 12 aand the second region 12 b thereunder is formed on the intermediatelayer 11.

In the compound semiconductor material, an energy gap increases(decreases) as an Al concentration in the composition increases(decreases). In the embodiment, the first region 12 a has a largerenergy gap than that of the intermediate layer 11, and the second region12 b has a smaller energy gap than that of the intermediate layer 11.Because of this configuration, a detection sensitivity of infrared raysof the QDIP is improved.

Subsequently, as illustrated in FIG. 2A, the quantum dots 14 are formed.

In detail, a substrate temperature is set to, for example, approximately470° C., and InAs having a thickness thereof corresponding to, forexample, a 2 or 3 monolayer is supplied. Here, in the initial stage ofInAs supply, InAs grows to be two-dimensionally and forms a wet layer.After that, in the subsequent supply of InAs, InAs three-dimensionallygrows in an island shape due to the strain attributed to a difference ofa lattice constant between AlAs and InAs, and the quantum dots 14 areself-assembled. These quantum dots 14 have respectively a diameter ofapproximately 10 nm to 20 nm and a height of approximately 1 nm to 2 nm,and the quantum dots are present approximately 10¹¹ dots/cm² in arealdensity. A part of underlying materials is incorporated into quantumdots. However, since the second region 12 b having a lower Alconcentration (here, Al is not contained) than that of the intermediatelayer 11 is provided between the first region 12 a and the intermediatelayer 11, a concentration of Al that is incorporated into the quantumdots 14 is suppressed. As a result, an impurity level that is formed inthe quantum dots 14 is suppressed.

Subsequently, as illustrated in FIG. 2B, a second barrier layer 15 isformed to cover the quantum dots 14.

In detail, in a state where the substrate temperature is maintained at,for example, approximately 470° C., a compound semiconductor materialhaving a composition with a higher Al concentration than that of theintermediate layer 11, for example, Al_(d)Ga_(1-d)As in which b<c(0.2<d≤1), here, AlAs not containing Ga (d=1) is grown to cover thequantum dots 14. Accordingly, the second barrier layer 15 is formed.

As seen from the above, the intermediate layer 11, the second region 12b of the first barrier layer 13, the first region 12 a of the firstbarrier layer 13, the quantum dots 14, and the second barrier layer 15are sequentially stacked so as to form a structure 10.

Subsequently, a series of processes of FIGS. 1B to 2B is repeatedlyperformed 10 times to 20 times, for example. Accordingly, as illustratedin FIG. 2C, a quantum dot structure 3 is formed by stacking a pluralityof (only five layers are illustrated in illustrated example) thestructures 10.

Subsequently, as illustrated in FIG. 3A, an upper contact layer 4 isformed on the quantum dot structure 3.

In detail, for example, Si doped n-type GaAs having an electronconcentration of 1×10¹⁸/cm³ is grown to a thickness of approximately 150nm. Accordingly, the upper contact layer 4 is formed on the quantum dotstructure 3.

Subsequently, as illustrated in FIG. 3B, a lower electrode 5 and anupper electrode 6 are formed.

In detail, first, an upper electrode is selectively etched using aresist mask or the like until a surface of the lower contact layer 2 isexposed through the upper contact layer 4. Here, a predetermined etchingstopper layer is formed between the lower contact layer 2 and thequantum dot structure 3, and the layer may be used as an etching stopperat the time of performing the selective etching described above.

Next, a resist mask including openings through which electrode formationparts are respectively exposed is formed on the lower contact layer 2and on the upper contact layer 4, and an electrode material such asAuGe/Ni/Au is deposited to fill the respective opening. The resist maskand AuGe/Ni/Au thereon are removed by lift-off. Accordingly, the lowerelectrode 5 is formed on the lower contact layer 2, and the upperelectrode 6 is formed on the upper contact layer 4.

Accordingly, the infrared detector according to the embodiment can beobtained.

Hereinafter, an effect exerted by the infrared detector according to theembodiment will be described based on comparison with a comparativeexample not including a second region of the first barrier layer. FIGS.4A and 4B are schematic sectional views for describing onlyincorporation of Al in a forming process of the quantum dots, FIG. 4Aillustrates the comparative example, and FIG. 4B illustrates theembodiment. FIG. 5 is a characteristic diagram illustrating S/N ratiosof the comparative example and the embodiment.

In the comparative example, as illustrated in FIG. 4A, a first barrierlayer 102 of AlAs is formed on an intermediate layer 101 of AlGaAs, andquantum dots 103 are self-assembled on the first barrier layer 102 inthe same manner as the embodiment. The quantum dots 103 are formed byassembling peripheral materials thereof in a winding manner therein asillustrated by an arrow B in the drawing. That is, Al contained in theintermediate layer 101 and the first barrier layer 102 is significantlyand chemically active, and as illustrated by arrows Al and A2 in thedrawing, impurities such as oxygen are incorporated into the quantumdots 103 with Al. Accordingly, an impurity level is formed in thequantum dots 103. When the impurity level is formed, noise at the timeof detecting infrared rays increases, and a S/N ratio is deteriorated.

In the embodiment with respect to that, as illustrated in FIG. 4B, thesecond region 12 b is provided between the intermediate layer 11 and thefirst region 12 a of the first barrier layer 13 as a buffer layer. Whenthe second region 12 b having a lower Al concentration than that of theintermediate layer 11 is present, the incorporation of Al to the quantumdots 14 is suppressed, and the impurities are reduced. Accordingly,formation of an impurity level in the quantum dots 14 is suppressed, andas illustrated in FIG. 5, noise at the time of detecting infrared raysis more reduced than that of the comparative example, and the S/N ratiois improved.

As described above, according to the embodiment, a QDIP with highreliability which is capable of reducing noise and improving a S/N ratiois realized.

Second Embodiment

Hereinafter, a second embodiment will be described. In this embodiment,in the same manner as the first embodiment, a QDIP is disclosed as aninfrared detector; however, the second embodiment is different in that acomposition of the first barrier layer is different from that of thefirst embodiment.

FIGS. 6A to 7B are schematic sectional views illustrating a main processin order of processes in a manufacturing method of the QDIP in thesecond embodiment.

First, in the same manner as the first embodiment, various processes ofFIGS. 1A and 1B are performed. At this time, the intermediate layer 11is formed on the lower contact layer 2.

Subsequently, as illustrated in FIG. 6A, a first barrier layer 22 isformed.

In detail, first, a compound semiconductor material having a compositionwith a lower Al concentration than that of the intermediate layer 11,for example, In_(a)Ga_(1-a)P (0≤a≤1) which has a lattice that matchesthat of GaAs and does not contain Al, here, In_(0.48)Ga_(0.52)P (a=0.48)is grown on the intermediate layer 11 to a thickness of, for example,approximately 0.3 nm. Accordingly, a second region 21 b is formed.

Next, a compound semiconductor material having a composition with ahigher Al concentration than that of the intermediate layer 11, forexample, Al_(c)Ga_(1-c)As in which b<c (0.2<c≤1), here, AlAs which doesnot contain Ga (c=1) is grown to a thickness of, for example,approximately 2 nm or less, here, approximately 0.3 nm, on the secondregion 21 b. Accordingly, the first region 21 a is formed. When thefirst region 21 a is formed to a thickness of 2 nm or less, an influenceof the incorporation of Al on the quantum dots 14 to be described lateris lessened.

Accordingly, the first barrier layer 22 including the first region 21 aand the second region 21 b thereunder is formed on the intermediatelayer 11.

In the compound semiconductor material, an energy gap increases(decreases) as an Al concentration in the composition increases(decreases). In the embodiment, the first region 21 a has a greaterenergy gap than that of the intermediate layer 11. Also, the secondregion 21 b has a lower Al concentration than that of the intermediatelayer 11. With this configuration, a detection sensitivity of infraredrays of the QDIP is improved.

Subsequently, as illustrated in FIG. 6B, the quantum dots 14 are formed.

In detail, a substrate temperature is set to, for example, approximately470° C., and InAs having a thickness corresponding to a 2 or 3 monolayeris supplied. Here, in the initial stage of InAs supply, InAs grows to betwo-dimensionally and forms a wet layer. After that, in the subsequentsupply of InAs, InAs three-dimensionally grows in an island shape due tothe strain attributed to a difference of a lattice constant between AlAsand InAs, and the quantum dots 14 are self-assembled. These quantum dots14 have respectively a diameter of approximately 10 nm to 20 nm and aheight of approximately 1 nm to 2 nm, and the quantum dots are presentapproximately 10¹¹ dots/cm² in areal density. A part of underlyingmaterials is incorporated into quantum dots. However, since the secondregion 21 b having a lower Al concentration (here, Al is not contained)than that of the intermediate layer 11 is provided between the firstregion 21 a and the intermediate layer 11, a concentration of Al that isincorporated into the quantum dots 14 is suppressed. As a result, animpurity level that is formed in the quantum dots 14 is suppressed.

Subsequently, as illustrated in FIG. 6C, the second barrier layer 15 isformed to cover the quantum dots 14.

In detail, in a state where the substrate temperature is maintained at,for example, approximately 470° C., a compound semiconductor materialhaving a composition with a higher Al concentration than that of theintermediate layer 11, for example, Al_(d)Ga_(1-d)As in which b<d(0.2<d≤1), here, AlAs not containing Ga (d=1) is grown to cover thequantum dots 14. Accordingly, the second barrier layer 15 is formed.

As seen from the above, the intermediate layer 11, the second region 21b of the first barrier layer 22, the first region 21 a of the firstbarrier layer 22, the quantum dots 14, and the second barrier layer 15are sequentially stacked so as to form a structure 20.

Subsequently, a series of processes of FIGS. 1B and 6A to 6C isrepeatedly performed 10 times to 20 times, for example. Accordingly, asillustrated in FIG. 7A, a quantum dot structure 7 is formed by stackinga plurality of (only five layers are illustrated in illustrated example)the structures 20.

Subsequently, as illustrated in FIG. 7B, the upper contact layer 4, thelower electrode 5, and the upper electrode 6 are formed.

In detail, first, for example, Si doped n-type GaAs having an electronconcentration of 1×10¹⁸/cm³ is grown to a thickness of approximately 150nm. Accordingly, the upper contact layer 4 is formed on the quantum dotstructure 7.

Next, an upper electrode is selectively etched using a resist mask orthe like until a surface of the lower electrode layer 2 is exposedthrough the upper contact layer 4. Here, a predetermined etching stopperlayer is formed between the lower electrode layer 2 and the quantum dotstructure 7, and the layer may be used as an etching stopper at the timeof performing the selective etching described above.

Next, a resist mask including openings through which electrode formationparts are respectively exposed is formed on the lower contact layer 2and on the upper contact layer 4, and an electrode material such asAuGe/Ni/Au is deposited to fill the respective opening. The resist maskand AuGe/Ni/Au thereon are removed by lift-off. Accordingly, the lowerelectrode 5 is formed on the lower contact layer 2, and the upperelectrode 6 is formed on the upper contact layer 4.

Accordingly, the infrared detector according to the embodiment can beobtained.

According to the embodiment, a QDIP with high reliability which iscapable of reducing noise and improving a S/N ratio is realized.

Third Embodiment

Hereinafter, a third embodiment will be described. In this embodiment,in the same manner as the first embodiment, a QDIP is disclosed as aninfrared detector; however, the third embodiment is different in that astructure of the second barrier layer is different from that of thefirst embodiment.

FIGS. 8A to 9 are schematic sectional views illustrating a main processin order of processes in a manufacturing method of the QDIP in the thirdembodiment.

First, in the same manner as the first embodiment, various processes ofFIGS. 1A to 2A are performed. At this time, the quantum dots 14 areformed on the first barrier layer 13.

Subsequently, as illustrated in FIG. 8A, a second barrier layer 32 isformed so as to cover the quantum dots 14.

In detail, first, in a state in which the substrate temperature ismaintained at, for example, approximately 470° C., a compoundsemiconductor material having a composition with a higher Alconcentration than that of the intermediate layer 11, for example,Al_(d)Ga_(1-d)As in which b<d (0.2<d≤1), here, AlAs not containing Ga(d=1) is grown to cover the quantum dots 14. Accordingly, a third region31 a is formed.

Next, a compound semiconductor material having a composition with alower Al concentration than that of the intermediate layer 11, forexample, Al_(e)Ga_(1-e)As in which e<b (0≤e<0.2), here, GaAs which doesnot contain Al (e=0) is grown to a thickness of, for example,approximately 0.3 nm on the third region 31 a. Accordingly, a fourthregion 31 b is formed. The third region 31 a and the fourth region 31 bthereon constitute the second barrier layer 32.

As seen from the above, the intermediate layer 11, the second region 12b of the first barrier layer 13, the first region 12 a of the firstbarrier layer 13, the quantum dots 14, and the third region 31 a of thesecond barrier layer 32, and the fourth region 31 b of the secondbarrier layer 32 are sequentially stacked so as to form a structure 30.

Subsequently, a series of processes of FIG. 1B to FIG. 2A and FIG. 8A isrepeatedly performed 10 times to 20 times, for example. Accordingly, asillustrated in FIG. 8B, a quantum dot structure 8 is formed by stackinga plurality of (only five layers are illustrated in illustrated example)the structures 30.

Subsequently, as illustrated in FIG. 9, the upper contact layer 4, thelower electrode 5, and the upper electrode 6 are formed.

In detail, first, for example, Si doped n-type GaAs having an electronconcentration of 1×10¹⁸/cm³ is grown to a thickness of approximately 150nm. Accordingly, the upper contact layer 4 is formed on the quantum dotstructure 8.

Next, an upper electrode is selectively etched using a resist mask orthe like until a surface of the lower contact layer 2 is exposed throughthe upper contact layer 4. Here, a predetermined etching stopper layeris formed between the lower contact layer 2 and the quantum dotstructure 8, and the layer may be used as an etching stopper at the timeof performing the selective etching described above.

Next, a resist mask including openings through which electrode formationparts are respectively exposed is formed on the lower contact layer 2and on the upper electrode layer 4, and an electrode material such asAuGe/Ni/Au is deposited to fill the respective opening. The resist maskand AuGe/Ni/Au thereon are removed by lift-off. Accordingly, the lowerelectrode 5 is formed on the lower contact layer 2, and the upperelectrode 6 is formed on the upper contact layer 4.

Accordingly, the infrared detector according to the embodiment can beobtained.

In the embodiment, in addition to the second region 12 b of the firstbarrier layer 13, the fourth region 31 b is provided between the thirdregion 31 a of the second barrier layer 32 and the intermediate layer 11thereon as a buffer layer. With this configuration, formation of theimpurity level in the quantum dots 14 is further suppressed, noise atthe time of detecting infrared rays is reduced, and a S/N ratio isimproved.

As described above, according to the embodiment, a QDIP with highreliability which is capable of reducing noise and improving a S/N ratiois realized.

Also, in the third embodiment, the second region 12 b of the firstbarrier layer 13 and the fourth region 31 b of the second barrier layer32 may be formed using InGaP instead of AlGaAs.

In addition, the second region of the first barrier layer in the firstto the third embodiments, and the fourth region of the second barrierlayer in the third embodiment may be formed using a mixed crystal of onekind or two or more kinds selected from the group consisting of GaAs,InAs, GaSb, InSb, GaP, and InP.

Fourth Embodiment

Hereinafter, a fourth embodiment will be described. In the embodiment,an infrared imaging device including one kind infrared detector selectedfrom the first to the third embodiments is disclosed.

FIG. 10 is a perspective view illustrating a schematic configuration ofthe infrared imaging device according to the fourth embodiment. FIG. 11is a schematic sectional view illustrating an enlarged part of theinfrared imaging device according to the fourth embodiment.

The infrared imaging device includes an infrared imaging panel 41 and areadout circuit 42, and the infrared imaging panel 41 and the readoutcircuit 42 are electrically connected to each other through a bump 43.

In the infrared imaging panel 41, one kind infrared detector selectedfrom the first to the third embodiments, here, for example, a pluralityof the infrared detectors 40 according to the first embodiment aredisposed in a plane in a matrix shape. Each infrared detector 40 becomesa pixel. The GaAs substrate 1, for example, an etching stopper layer 33of InGaP and the lower contact layer 2 in each infrared detector 40 arecommonly used in the infrared imaging panel 41. A wiring 44 is formed onan insulator 34 of each infrared detector 40, and the bump 43 is formedon each wiring 44. A part of the bump 43 is used as a common electrode43 a of each infrared detector 40. An electrode 5 or 6 which is notillustrated is electrically connected to one end of each wiring 44(through an opening 34 a formed on the insulator 34, for example).

The readout circuit 42 is a driving unit of each infrared detector 40,and includes a plurality of transistors 45 and power supply lines 46 forapplying a power supply voltage V_(A). Each transistor 45 iselectrically connected to the bump 43, and each power supply line 46 iselectrically connected to the common electrode 43 a. When the powersupply voltage V_(A) is applied, an output current flows to eachinfrared detector 40.

According to the embodiment, an infrared imaging device with highreliability including an infrared detector 40 with high reliabilitywhich is capable of reducing noise and improving a S/N ratio isrealized.

Fifth Embodiment

Hereinafter, a fifth embodiment will be described. In the embodiment, aninfrared imaging system including an infrared imaging device accordingto the fifth embodiment is disclosed.

FIG. 12 is a schematic diagram illustrating a schematic configuration ofan infrared imaging system according to the fifth embodiment.

The infrared imaging system includes a sensor unit 51, a controlcalculation unit 52, and a display unit 53.

The sensor unit 51 includes a lens 54 collecting incident light, aninfrared imaging device 50 according to the fourth embodiment, and acooling unit 55 for cooling the infrared imaging device 50. The controlcalculation unit 52 controls the driving circuit 42 of the infraredimaging device 50. The display unit 53 displays an infrared image whichis captured by the infrared imaging device 50 based on an imagecapturing signal transmitted from the control calculation unit 52.

According to the embodiment, an infrared imaging system with highreliability including an infrared detector 40 with high reliabilitywhich is capable of reducing noise and improving a S/N ratio isrealized.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An infrared detector comprising: a quantum dotstructure; and an electrode that is coupled to the quantum dotstructure, wherein the quantum dot structure is obtained by stacking aplurality of structures each including a quantum dot, a first barrierlayer under the quantum dot and a second barrier layer over the quantumdot to cover the quantum dots, and an intermediate layer under the firstbarrier layer, and wherein the first barrier layer includes a firstregion and a second region having a lower Al concentration than that ofthe intermediate layer between the first region and the intermediatelayer.
 2. The infrared detector according to claim 1, wherein the firstregion has a larger energy gap than that of the intermediate layer, andthe second region has a smaller energy gap than that of the intermediatelayer.
 3. The infrared detector according to claim 1, wherein the secondregion has Al_(a)Ga_(1-a)As (0≤a<1).
 4. The infrared detector accordingto claim 1, wherein the second region has In_(a)Ga_(1-a)P (0≤a≤1). 5.The infrared detector according to claim 3, wherein the intermediatelayer has Al_(b)Ga_(1-b)As (0<b≤1), and the first region hasAl_(c)Ga_(1-c)As (0<c≤1), where b<c.
 6. The infrared detector accordingto claim 5, wherein the second barrier layer has Al_(d)Ga_(1-d)As(0<d≤1), where b<d.
 7. The infrared detector according to claim 1,wherein the second region has a mixed crystal of one kind or two or morekinds selected from the group consisting of GaAs, InAs, GaSb, InSb, GaP,and InP.
 8. The infrared detector according to claim 1, wherein thesecond barrier layer includes a third region and a fourth region havinga lower Al concentration than that of the intermediate layer over anupper portion of the third region.
 9. The infrared detector according toclaim 1, wherein the first region has a thickness of 2 nm or less. 10.An imaging device comprising: a plurality of infrared detectors; and areadout circuit that drives the infrared detectors, wherein the infrareddetector includes a quantum dot structure, and an electrode that iscoupled to the quantum dot structure, wherein the quantum dot structureis obtained by stacking a plurality of structures each including aquantum dot, a first barrier layer under the quantum dot and a secondbarrier layer over the quantum dot to cover the quantum dots, and anintermediate layer under the first barrier layer, and wherein the firstbarrier layer includes a first region and a second region having a lowerAl concentration than that of the intermediate layer between the firstregion and the intermediate layer.
 11. The imaging device according toclaim 10, wherein the second region has Al_(a)Ga_(1-a)As (0≤a<1). 12.The imaging device according to claim 10, wherein the second region hasIn_(a)Ga_(1-a)P (0≤a≤1).
 13. An imaging system comprising: an infraredsensor assembly; a controller that controls the infrared sensor unit;and a display that displays a captured infrared image, wherein theinfrared sensor assembly includes an infrared imaging device, a coolerthat cools the infrared imaging device, and a lens for incident infraredrays to the infrared imaging device, wherein the infrared imaging deviceincludes a plurality of infrared detectors, and a readout that drivesthe infrared detectors, wherein the infrared detector includes a quantumdot structure, and an electrode that is coupled to the quantum dotstructure, wherein the quantum dot structure is obtained by stacking aplurality of structures each including a quantum dot, a first barrierlayer under the quantum dot and a second barrier layer over the quantumdot to cover the quantum dots, and an intermediate layer under the firstbarrier layer, and wherein the first barrier layer includes a firstregion and a second region having a lower Al concentration than that ofthe intermediate layer between the first region and the intermediatelayer.
 14. The imaging system according to claim 13, wherein the secondregion has Al_(a)Ga_(1-a)As (0≤a<1).
 15. The imaging system according toclaim 13, wherein the second region has In_(a)Ga_(1-a)P (0≤a≤1).